Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 182342
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
642 PM CDT SAT APR 18 2015

.SHORT TERM...(THIS EVENING THROUGH SUNDAY)
ISSUED AT 256 PM CDT SAT APR 18 2015

19Z SURFACE ANALYSIS INDICATING A WEAK FRONTAL BOUNDARY LOCATED JUST
TO OUR NORTH WITH WINDS BACKING MORE INTO A EAST TO NORTHEAST
DIRECTION BEHIND THE BOUNDARY AND DEW POINTS LOWERING INTO THE
MIDDLE 40S. SOUTH OF THE BOUNDARY...TEMPERATURES WERE IN THE UPPER
70S TO LOWER 80S WITH DEW POINTS STILL IN THE COMFORTABLE RANGE...
MOSTLY THE MIDDLE 50S. OVER THE PAST HOUR THERE HAVE BEEN SOME
ISOLATED THUNDERSTORMS THAT DEVELOPED ALONG THE WEAK CONVERGENCE
ASSOCIATED WITH THE FRONTAL BOUNDARY WITH A GENERAL DRIFT TO THE
NORTH NOTED. MODELS SUGGEST MOST OF THE SHOWERS ASSOCIATED WITH
THE FRONT JUST TO OUR NORTH SHOULD DISSIPATE THIS EVENING WITH
A MUCH LARGER AND MORE ORGANIZED BAND OF SHOWERS AND STORMS LOCATED
TO OUR WEST ASSOCIATED WITH THE 500 MB TROF THAT WAS FORECAST TO
OPEN UP AND SHIFT EAST INTO THE MIDWEST ON SUNDAY.

MAIN FORECAST CHALLENGE TONIGHT WILL BE TIMING OF THE PRECIP
ASSOCIATED WITH THE 500 MB TROF TO OUR WEST WITH THE HIGHER POPS
CONFINED TO WESTERN IL AND LATER TONIGHT OVER SOUTHEAST IL AS A
LOBE OF VORTICITY SWINGS NORTH INTO THE I-70 BY SUNDAY MORNING.
FURTHER NORTH...AFTER OUR SLIGHT CHANCES END IN THE FAR NORTH
EARLY THIS EVENING...NOT LOOKING FOR MUCH IN THE WAY OF RAIN UNTIL
SUNDAY MORNING AS THE MAIN AREA OF LIFT AND DEEPER MOISTURE STREAMS
NORTH AHEAD OF THE AFOREMENTIONED VORT MAX.

.LONG TERM...(SUNDAY NIGHT THROUGH SATURDAY)
ISSUED AT 259 PM CDT SAT APR 18 2015

A WEAK SURFACE LOW WILL PROGRESS FROM SW TO NE ACROSS EASTERN IL ON
SUNDAY, IN RESPONSE TO THE CLOSED LOW OVER COLORADO FINALLY BEING
ABSORBED INTO THE NORTHERN STREAM. A RAPID INCREASE IN ISENTROPIC
LIFT AND MOISTURE ADVECTION WITH THE ARRIVAL OF THE LOW WILL CAUSE
RAIN CHANCES TO INCREASE FROM SOUTH TO NORTH SUNDAY MORNING. WE
EXPECTED HIGHER RAINFALL TOTALS CLOSER TO THE TRACK OF THE LOW UP
THROUGH EASTERN IL, BUT MOST AREAS SHOULD SEE RAIN AT SOME POINT ON
SUNDAY. ELEVATED INSTABILITY LOOKS SUFFICIENT FOR CONVECTIVE
UPDRAFTS AND THUNDERSTORM DEVELOPMENT, ESPECIALLY SUNDAY AFTERNOON
EAST OF I-55 JUST AHEAD OF THE LOW CENTER. SPC MARGINAL RISK COVERS
OUR SOUTHERN AREA ROUGHLY SOUTH OF I-70 DURING THAT TIME. SOME HAIL
AND STRONG WINDS WILL BE THE PRIMARY HAZARDS.

THE LOW SHOULD DEPART INTO SOUTHERN LOWER MICHIGAN SUNDAY EVENING.
THE UPPER TROUGH AXIS WILL REMAIN JUST WEST OF IL SUNDAY NIGHT, WITH
UPPER LEVEL ENERGY STILL POISED TO TRIGGER POST-LOW SHOWERS. WE KEPT
LIKELY POPS GOING SUNDAY NIGHT, BUT WESTERN AREAS SHOULD BEGIN TO
SEE DIMINISHING PRECIP BY 12Z/7AM MONDAY AS A STRONGER VORT MAX
DEPARTS INTO INDIANA. WE KEPT LOW CHANCE POPS EAST OF I-57 MONDAY
MORNING, WITH SLIGHT CHANCE POPS AS FAR WEST AS SPI/PIA. PRECIP
SHOULD COME TO AN END IN ALL OF OUR AREA MONDAY AFTERNOON, AS DRY
AIR ENTRAINS INTO THE UPPER TROUGH.

UPPER LEVEL RIDGING OVER EACH COAST OF THE LOWER 48 WILL CAUSE THE
UPPER LOW TO STALL OUT OVER THE WESTERN GREAT LAKES FOR MUCH OF THE
WEEK. HOWEVER, THE MAIN RESULT WILL BE COOLER WEATHER, PRECIP
CHANCES GENERALLY ON THE LOW SIDE.

ONE CHANCE FOR RAIN APPEARS TIED TO A COLD FRONT FOR TUES NIGHT.
THE MODELS HAVE COME INTO BETTER AGREEMENT, AND GFS/ECMWF/CANADIAN
ALL INDICATE THIS FEATURE, WITH THE GFS/ECMWF PRODUCING LIGHT
RAINFALL DURING THAT TIME. WE INTRODUCED SOME SLIGHT CHANCE POPS
TUES NIGHT, AS RAIN AMOUNTS WILL BE SPOTTY IN THE COOLER AND
GENERALLY DRY AIRMASS.

THE ONLY OTHER CHANCE OF RAIN FOR THE REST OF THE WEEK LOOKS TO COME
FRIDAY NIGHT WITH A SOUTHERN STREAM WAVE THAT THE ECMWF BRINGS FAR
ENOUGH NORTH TO AFFECT OUR COUNTIES SOUTH OF I-72. THE GFS KEEPS THE
AFFECTS FROM THE SYSTEM SOUTH OF IL. LOW CHANCE POPS WERE INCLUDED
FRI NIGHT IN OUR SOUTHERN AREA FOR NOW.

COOLER AIR WILL BE THE MAIN WEATHER CONCERN FOR THE REMAINDER OF THE
WEEK, AS THE BLOCKED PATTERN KEEPS THE UPPER LOW ACROSS THE MID-
STATES. COOL NW FLOW WILL HELP TO HIGHS MAINLY IN THE UPPER 50S
TO LOWER 60S AND LOWS IN THE UPPER 30S TO LOW 40S. WE CAN NOT RULE
OUT PATCHY FROST IN OUR COLDER NORTHERN SPOTS WED NIGHT AND THUR
NIGHT IF SKIES CLEAR OUT AND WINDS BECOME LIGHT.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z SUNDAY EVENING)
ISSUED AT 642 PM CDT SAT APR 18 2015

MAINLY VFR CONDITIONS EXPECTED THROUGH CENTRAL IL UNTIL AROUND
12Z WHEN A SURFACE LOW DEVELOPS AND LIFTS NORTHEASTWARD INTO THE
AREA WITHIN A LARGE SCALE UPPER TROUGH PATTERN OVER THE CENTRAL
U.S. ISOLATED THUNDERSTORMS ARE A REMOTE POSSIBILITY KPIA-KSPI
WESTWARD THIS EVENING...BUT LIKELIHOOD NOT HIGH ENOUGH FOR MENTION
IN 00Z TAFS. EXPECT INCREASING PRECIPITATION AT SOUTHERN TERMINALS
KSPI-KDEC AROUND 10Z SPREADING NORTH TO KPIA-KBMI-KCMI AROUND 12Z.
BY 13-15Z...PREDOMINANT RAIN WITH MVFR CIGS/VSBYS LIKELY AND INCLUDED
IN TAFS. ISOLATED THUNDERSTORMS POSSIBLE AS WELL...BUT
LIKELIHOOD/TIMING NOT HIGH ENOUGH FOR MENTION THIS FAR OUT. WINDS
E6-12 KTS THROUGH 15Z SHIFTING TO N-NW DURING AFTERNOON AS LOW
LIFTS TO THE EAST OF CENTRAL IL AREA.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...SMITH
LONG TERM...SHIMON
AVIATION...ONTON


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